I'm not suggesting Facebook is unchallengeable just that it will take much longer due to the enormous number of users. The exodus from MySpace took several year

I'm not suggesting Facebook is unchallengeable just that it will take much longer due to the enormous number of users. The exodus from MySpace took several years with about 1/4th less users. So a 10 year timeframe seems completely reasonable to me when you're talking about almost 700 million users. I just can't see how that happens in 4-6 years as you suggest. Facebook would have to stop growing entirely and begin losing hundreds of thousands of users per day to a competitor.